Rear mag problem

[KyleTownsend]

I recently had this problem on a rebuilt mag from Kelley Aerospace. The problem was that the tach breaker points were improperly assembled. An insulating washer was in the wrong place. This caused a dead short across the tach points no matter whether the points were open or not. Easy to check. Just test continuity across the two tach leads from the mag while slowly rotating the prop by hand. They should alternate between open and closed circuit. If not, then the problem is with the tach points. If this tests OK, then the problem is somewhere between the tach and the mag (probably the previously mentioned connectors).